linux-kernel - Re: [GIT PULL] tracing: Updates for 5.18

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Date:   Wed, 23 Mar 2022 11:47:07 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        Tom Zanussi <zanussi@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [GIT PULL] tracing: Updates for 5.18

On Mon, Mar 21, 2022 at 7:56 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> A restructure of include/trace caused a conflict [..]

Ugh. Disgusting. And in the very same pull request it shows why that
TRACE_CUSTOM_EVENT() thing that caused this all was actually a
horribly bad idea, since it also then DIDN'T WORK due to the kernel
interfaces changing.

So this restructuring seems to have been triggered by something that
was a bad idea to begin with.

But the real problem is here:

> Tracing updates for 5.18:
>
> - New user_events interface. User space can register an event with the kernel
>   describing the format of the event. Then it will receive a byte in a page
>   mapping that it can check against. A privileged task can then enable that
>   event like any other event, which will change the mapped byte to true,
>   telling the user space application to start writing the event to the
>   tracing buffer.

That explanation makes no sense, because it doesn't actually explain *why*.

It explains *what*, but the big issue for new interfaces shoudl always
be why the heck a new interface was needed in the first place.

I've pulled this, but under protest.

              Linus